The Sheaffer Flat Tops introduced in 1912 were made from hard rubber but it wasn’t until 1924 that the first plastic ones were produced. If my research is correct, this material was Radite, a partially nitrated celluloid.
Jade green was one of the more popular colors and when new were quite stunning. Barrels often discolored and it it thought that vapors given off from the rubber sacs may have caused this. The clip on this pen, the Junior, was used between 1922 and 1927 when the hump clip was introduced.
On this pen the barrel imprints have hyphens rather that periods which again date it to before 1927 and as the white dot was introduced in 1923 it seems that this pen was manufactured between 1923 and 1927. Although the feed looks to be original, I suspect the nib is of a later date.